Parts #19-24 in this diagram.
I am not sure what year/size engine you have though.
It's located in the back of the engine, bottom of the power head.
You'll see a 1+- inch hose with a cable tie on it that connects to the poppit valve cover.
It's not all that difficult to replace, I also think the new parts have been redesigned and will look a bit different than the one you take out.
When I did mine, it was out of the water and I think it'll be alot more difficult if you tried this with the boat in.
A swivel socket will make the job easier as will another pair of hands.
I took the ECU off and slid it out of the way but did not disconnect any of the wires from it.
I do not think a weak pee stream would be indicative of a bad poppit valve, though I am no expert. Others will chime in, good luck.
http://www.boats.net/parts/search/Yamah ... parts.html
